Changes
- Channels: bi-directional communication between tasks and runtimes
- Improvements to copying making it vastly more efficient
- Improvements to runtime stack, making it more efficient
- Tasks may use lexical scope
- Events - an experimental, quite primitive event loop for reading/writing sets of channels and futures
- Support for rethrowing exceptions uncaught in tasks
- Simplified Runtime constructor
- Remove Future::select (in favour of the superior events interface)
- Task cancellation
- Closure support
- Drop support for PHP 7.1